1. Field of the Invention
The present invention relates to microwave device test fixtures in general and in particular to a coaxial test fixture for testing carrier-mounted and non-carrier-mounted microstrip and coplanar devices.
2. Description of the Prior Art
Microwave device test fixtures to which the present invention relates are generally used for testing microstrip and coplanar devices before they are mounted in a permanent housing.
A conventional microstrip device comprises a thin fragile substrate. The substrate typically comprises a ceramic material, such as alumina. An electrically conductive material, such as gold, is plated on one surface, i.e. top surface, of the substrate and then etched to form one or more electrical conductors thereon. Various types of circuit devices may also be mounted on the top of the substrate and connected to the conductors thereon. An electrically conductive material, such as gold, is also plated on the opposite surface, i.e. bottom, of the substrate so as to provide a ground plane for the device circuits. For some applications the substrate is then mounted on a relatively thicker block of metal which increases the thickness of the ground plane and serves as a carrier for the substrate. Such devices are commonly called carrier mounted devices. In some applications the carrier is not required or used.
Coplanar devices are similar to microstrip devices except that the ground plane is located on the top of the substrate adjacent to, but spaced from, the center conductor.
In practice, the conductors on the top of the substrate, the ground plane and the carrier, when used, extend to the edge of the substrate where they are available to be connected to other microwave components, such as other microstrip circuits, in a microwave device or to connectors in a microwave device test fixture.
Conventional microwave device test fixtures of the type used for testing microstrip and coplanar devices typically comprise a pair of upstanding spaced metallic blocks in each of which is mounted a coaxial microwave RF connector having an outer conductor and a center conductor. A base member is provided between the blocks for supporting the microwave device under test (DUT), i.e. the stripline or coplanar device. In some fixtures, the base member is permanently installed. In others, the base member is removable. Whether permanently installed or removable, the size of the DUT, i.e. its length, width and thickness, is typically limited by the size of the base member on or in which the DUT is mounted for testing. Therefore, to accommodate a range of device sizes, an equal number of specially designed and sized base members is usually required.
To test a microwave device, such as a microstrip, in a conventional test fixture having a base member as described above, it has been the practice to mount the strip on the base member so as to permit an electrical connection between the ground plane and circuit of the device and the outer and center conductors of the coaxial connectors in the fixture, respectively. In many such fixtures, the RF connectors are axially aligned and rigidly fixed in position so that relative transverse movement of the connectors is not possible, thus restricting use of the fixture to testing microstrip having axially aligned circuit terminals.
In certain ones of the above-described prior known fixtures, the connection to the ground plane is made through the bottom of the carrier while the connection to the microstrip circuit is made by forcing the center conductor of the coaxial connector or an extension thereof against the microstrip circuit. A disadvantage of this type of fixture is that the ground path between the outer conductor of the fixture connectors and the ground plane of the microstrip through the carrier is undesirably long and thus excessively inductive.
A pair of prior known fixtures comprising a means for reducing the length of the ground path are described in an article by P. J. Lang et al, Microwave Journal, May, 1988 at page 365. In these fixtures, a close connection with the ground plane is made through a pressure contact which is forced axially against an upper edge of the carrier below the substrate near the ground plane. While effectively reducing the length of the ground path, disadvantages of these fixtures are that the fixtures can be used only for testing carrier-mounted microstrip devices and must provide a means for applying orthogonal as well as axial pressure to the device under test in order to make the necessary electrical connections therewith.
In general, prior known fixtures for testing microstrip and coplanar circuits have been limited to testing carrier-mounted circuits having axially aligned terminals, have required a base or other member for supporting the carrier-mounted circuit and have not been usable for testing more than one substrate at a time.